1. Field of the Invention
The invention relates to a press felt for the transportation of a web and for the dewatering thereof in a press part of a paper machine, said press felt comprising at least one needled flock layer of fibres.
2. Description of the Background
Press felts of this type are today extremely well-known. However, the use of this kind of conventional felts in a press has brought about difficult problems due to effects such as the blow effect and re-wetting. These problems result from the air and water contained in the surface layer of the felt. The blow effect is caused by the air contained in the pores of conventional press felts on the surface thereof and therewithin. When the felt is compressed in the press roll nip, the air is forced out of the surface of the felt through large pores thereof so that the web is lifted off the felt. The re-wetting, in turn, is due to the fact that the felt and the web make contact with each other after the nip effected by the press roll and thus the water contained in the felt can be returned to the web. The re-wetting is aggravated by the presence of large pores and capillaries of the felt surface. These problems set limits to the speed of the paper machine and the linear pressure of the press.
Another disadvantage of needled felts is that the contact face between the web and the felt in the press is uneven because of the irregular formation of the flock layer.
Before the introduction of needled felts, attempts were made to solve this kind of blow and rewitting problems by weaving the surface layer more closely than the layer below it. An example of the prior art would be the structure disclosed in U.S. Pat. No. 3,214,326. However the solution according to U.S. Pat. No. 3,214,326 is not at all applicable to a press provided with needled felts, but only to felts with woven structures.